As part of release, generate a file containing multiple digests

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Description

Currently as part of the release we generate .md5 and .sha1 digests (as well as the pgp .asc file) and the download page http://calcite.apache.org/downloads/ references the md5 and pgp but not the sha1.

Per http://www.apache.org/dev/release-signing.html#md5-security md5 is no longer secure, and sha512 is preferred over sha256. The best approach seems to be to generate multiple digests, and generate new ones as best practices change. I think we should generate checksum file with a .mds suffix as follows: